This is usually an error caused by using an absolute path in SRCS. This
happened to me in 83c20b8a2da0 due to changing LDADD to SRCS.
I did not notice that this had created a .o file inside the source tree
since .gitignore contains "*.o" and therefore git did not report any
Adding this warning message to bsd.lib.mk/bsd.prog.mk should prevent
issues like this in the future.
There is one case of an absolute OBJS path in the current source tree
which I have tagged as such using _ABSOLUTE_PATH_OBJS.